1. Field of the Invention
The present invention relates to recovery of elemental sulfur from gas streams containing sulfur dioxide.
2. Description of the Prior Art
Flue gases emitted from burning sulfur containing fossil fuels are the most common dilute sulfur dioxide (SO.sub.2) containing industrial gases. The majority of commercial scale flue gas desulfurization (FGD) plants in use today for combustion gas purification are based on wet scrubbing processes. Many of them are of the "throwaway" type, fixing the sulfur in a solid waste product, which requires disposal. These FGD systems do not recover elemental sulfur. However, several other wet and dry FGD processes are of the regenerative type combining sulfur dioxide removal with active medium regeneration and concomitant sulfur recovery.
Many sulfur recovery methods have been proposed. Often the type and location of the primary operation (sulfur source) dictate the choice of the sulfur recovery method. For example, sulfur recovery from metallurgical operations (smelters, sulfide ore roasters) is typically in the form of sulfuric acid. On the other hand, petroleum refineries emit H.sub.2 S-rich gas streams which are processed in multi stage Claus plants to recover elemental sulfur.
Recovery of sulfur values in elemental sulfur form is more desirable than sulfuric acid or liquid SO.sub.2, as local market conditions are typically more restrictive for the latter. (See, for example, J. B. Rinckhoff, in J. B. Pfeiffer, (ed.), "Sulfur Removal and Recovery from Industrial Processes," Advances in Chemistry Series, No. 139, p. 48, American Chemical Society, 1975). For SO.sub.2 -containing industrial gases, this means reducing the SO.sub.2 with a gaseous reducing agent, such as carbon monoxide, hydrogen, synthesis gas (H.sub.2 +CO), or natural gas, or with a carbonaceous solid (such as activated charcoal, coke, anthracite coal). The Allied Chemical sulfur dioxide reduction technology employs a catalyst over which SO.sub.2 reduction by natural gas (CH.sub.4) takes place, producing a mixture of H.sub.2 S, elemental sulfur and (unconverted) SO.sub.2. After condensation of sulfur, further sulfur recovery is accomplished in two-stage Claus plants. This process requires relatively concentrated SO.sub.2 (&gt;4.0%) gases and downstream Claus plants to complete sulfur recovery. (See, for example, W. D. Hunter, Jr., "Reducing SO.sub.2 in Stack Gas to Elemental Sulfur," Power 117 (9), 63, 1973); U.S. Pat. Nos. 3,653,833 (April, 1972), and 3,755,551 (August, 1973)). The RESOX process, developed by the Foster Wheeler Energy Corporation, described in "The FW-BF SO.sub.2 Removal System," (Sulfur, No. 119, 24-26 and 45, July-Auqust 1975), partially reduces the SO.sub.2 -rich streams (&gt;10.0% SO.sub.2) to elemental sulfur and organosulfur compounds by reaction with coke at 850.degree.-900.degree. C. (See also, R. E. Rush, and R. A. Edwards, "Operating Experience with Three 20 MW Prototype Flue Gas Desulfurization Processes at Gulf Power Company's Scholtz Electric Generating Station," presented at EPA Flue Gas Desulfurization Symposium, Hollywood, Fla., Nov. 8-11, 1977).
Direct flue gas reduction by synthesis gas over an undisclosed catalyst is proposed by K. V. Kwong et al. in "The Parsons FGC Process Simultaneous Removal of SO.sub.x and NO.sub.x," (presented at the 1990 Annual Meeting of AIChE, Chicago, Ill., Nov. 11-16, 1990) to simultaneously reduce the oxygen, SO.sub.x and NO.sub.x in the flue gas. The H.sub.2 S produced is selectively recovered by solvents, concentrated and taken to multi-stage Claus plants for elemental sulfur recovery. This process does not achieve a single-step SO.sub.2 reduction to sulfur. Similarly, earlier proposed schemes of flue gas reduction could not achieve both high SO.sub.2 conversion as well as high selectivity to elemental sulfur in a single-stage catalytic reactor.
The catalytic removal of sulfur dioxide by carbon monoxide involves a main reaction producing elemental sulfur: ##EQU1## where x varies between 2 and 8, as well as a competing side reaction producing carbonyl sulfide: EQU CO+S=COS (2)
At about the stoichiometric ratio of CO/SO.sub.2 reaction (1) is favored, while excess CO increases production of COS.
R. P. Ryason et al. (Air Pollut. Contr. Ass. 17, 796, 1967), and U.S. Pat. No. 3,454,355 (July, 1969) reported on the use of single-bed catalysts (Cu, Pd, Ag, Co or Ni supported on alumina) to produce sulfur from dry sulfur dioxide gases. R. Querido and W. L. Short, and V. C. Okay and W. L. Short (Ind. Eng. Chem. Prod. Res. Develop. 12, 10 and 291, 1973) demonstrated the feasibility of reducing sulfur dioxide by carbon monoxide on a Cu-alumina catalyst at concentrations and temperatures typical of power plant stack gases; however, the catalyst activity was severely poisoned by water vapor. To address similar problems on iron-alumina catalysts, Khalafalla et al. (Ind. Eng. Chem. Prod. Res. Develop. 10(2), 133, 1971 (also in "Sulfur Removal and Recovery from Industrial Processes" supra, at 60) proposed using dual catalyst beds, the second bed serving as a Claus reactor which produces elemental sulfur by reacting a portion of the SO.sub.2 -feed stream with by-product H.sub.2 S released from the first catalyst bed.
Cobalt oxides have been examined by J. G. I. Bazes, L.S. Caretto and K. Nobe as catalysts for the reduction of sulfur dioxide with carbon monoxide (Ind. Eng. Chem. Prod. Res. Develop., 14(4), 264, 1975). The primary focus of that paper was to study the SO.sub.2 conversion over various cobalt oxide catalysts (LaCo.sub.3, CeO.sub.2 --Co.sub.3 O.sub.4, CuCo.sub.2 O.sub.4) selectively to carbonyl sulfide (COS) rather than elemental sulfur. At temperatures between 287.degree. and 381.degree. C. and CO/SO.sub.2 ratios between 3.64 and 5.68, the highest sulfur dioxide conversion disclosed in this article was 94% with selectivity to COS of 48%. More recently, D. B. Hibbert and R. H. Campbell in "Flue Gas Desulfurization: Catalytic Removal of Sulfur Dioxide by Carbon Monoxide on Sulfided La.sub.1-x Sr.sub.x CoO.sub.3 " (Applied Catalysis 41, 289, 1988), disclosed strontium doped lanthanum cobalt oxides to reduce sulfur dioxide with carbon monoxide. Even at near stoichiometric ratios of CO/SO.sub.2 however, the formation of COS continues to be a factor, limiting the elemental sulfur recovery.
In addition to power plant-SO.sub.2 emissions, dilute sulfur dioxide-containing gas streams are produced in waste incinerators, industrial furnaces, and by process equipment used in petroleum refineries and sulfuric acid plants, and spent sorbent or catalyst regenerator equipment. Sulfur recovery involves several steps, such as partial reduction of SO.sub.2 to H.sub.2 S, followed by Claus processing. No single-stage process presently exists to directly reduce the varying SO.sub.2 -effluent gases to elemental sulfur over a catalyst and which displays both high activity and high selectivity.
Cerium oxide formulations are known in the art as good absorbents of sulfur oxides (removal of So.sub.2 from oxygen-containing flue gases via conversion to solid sulfates that can be regenerated back to the oxide form is disclosed by J. M. Longo in "Process for Desulfurization of Flue Gas", U.S. Pat. No. 4,001,375 (1977)). Also, R. P. Cahn and J. M. Longo, "Desulfurization of Hot Gas with Cerium Oxide", U.S. Pat. No. 4,346,063 (1982) showed that cerium oxide catalyzes the oxidation of H.sub.2 S to SO.sub.2, which can then be removed by reacting with CeO.sub.2 (to form a sulfate). Therefore, CeO.sub.2 may be used for Claus plant tail gas treatment by adding oxygen in the gas. J. P. Brunelle et al., in "CeO Catalytic Desulfurization of Industrial Gases," U.S. Pat. No. 4,857,296 (August, 1989), disclose CeO.sub.2 as a good catalyst for the Claus reaction, 2H.sub.2 S+SO.sub.2 =3S+2H.sub.2 O, and for the hydrolysis of organosulfur compounds (COS and CS.sub.2) displaying an especially high activity for the latter.
The prior art however does not teach or suggest the use of cerium oxide formulations as catalysts for the direct elemental sulfur recovery from SO.sub.2 -containing industrial gas streams by reacting these gas streams with carbon monoxide or other reducing gases.